Many articles of manufacture employing heat seals are currently available in the marketplace. Generally, the seals on such articles may be employed by welding two separate portions of the article together. For example, plastic parts usefully employed in machines and toys may be constructed by joining together two individual plastic pieces by heating one or both of the plastic pieces, pressing them together, and then, allowing them to cool. Specifically, heat sealing is very important in packaging applications. Packages formed by a heat seal provide for the efficient transportation of a consumer item within the package, provide a display of the consumer item that promotes sales, and, in the food industry, the packaging is employed to preserve the freshness of the consumer item. Most importantly and related to heat seal a manufacturer of packages or any other like article requiring a seal requires excellent processibility.
Various types of polymers are used to form articles, which include packages, that may be joined together or sealed by the application of heat and/or pressure. Polymers or blends of polymers used to make the articles are selected for use because they provide a strong seal, which is easily and rapidly formed by a single short application of heat and/or pressure. Occasionally, the entire heat sealed article is constructed from the same polymer or a blend of polymers or by the coextrusion of the same or different polymers.
More often, the article is constructed of various areas or layers of different materials, and polymers which provide good heat sealing properties are utilized only in areas, or layers, where heat sealing will ultimately be necessary. This type of construction is employed because the articles, for instance multilayer films, should have desirable physical and mechanical properties such as clarity, strength, resistance to puncture and tearing, in addition to heat sealing properties, and should be easily processed by high speed equipment. Many plastic materials are known to possess good physical and mechanical properties but often do not also possess good heat sealing properties. For example, polypropylene has good strength and clarity and is resistant to tearing, but does not readily form good seals at the temperatures which are preferred in commercial sealing machinery. Conversely, some polymers with good heat sealing properties do not have adequate strength or clarity.
The packaging art has therefore developed multiple layer articles such as multilayer films incorporating one or more layers of the same or different types of polymers blended or coextruded together that provide good mechanical and physical properties and providing one or more additional layers formed from polymers that provide the article of manufacture with good heat sealing properties. In this way, for example, a film may be produced having a substrate layer of polypropylene provided for strength and clarity, and a layer of polyethylene to provide good heat sealing properties. Other articles, in addition to films, may be similarly constructed with a plurality of materials, each material selected to contribute to one or more of the desired properties of the final article.
Various types of polyethylene polymers are known in the art as having acceptable heat sealing properties. Low density polyethylene ("LDPE") is generally prepared at high pressure using free radical initiators and typically has a density in the range of 0.915-0.940 g/cm.sup.3. LDPE is also known as "branched" polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone.
High density polyethylene ("HDPE") usually has a density in the range of greater than 0.940 to 0.960 g/cm.sup.3. HDPE is prepared using a coordination catalyst, e.g., Ziegler-Natta type catalysts, at low or moderate pressures, but sometimes at high pressure. HDPE is generally linear without any substantial side chain branching. HDPE is a substantially crystalline polymer.
Linear low density polyethylene ("LLDPE") is generally prepared in the same manner as HDPE, but incorporates a relatively minor amount of an alpha-olefin comonomer such as butene, hexene or octene to introduce enough short chain branches into the otherwise linear polymer to reduce the density of the resultant polymer into the range of that of LDPE. The coordination catalysts used to interpolymerize ethylene and the alpha-olefins generally produce a LLDPE with a broad composition distribution, as hereinafter defined, and a relatively broad molecular weight distribution, i.e., Mw/Mn greater than about 3, wherein Mw is the weight average molecular weight and Mn is the number average molecular weight.
Polyethylenes such as LLDPE described above have a broad molecular weight distribution which can be undesirable in many respects. For example, LLDPE resins known previously in the art contain relatively high molecular weight molecules that are subject to an orientation, which results in anisotropic properties in the machine direction compared to the transverse direction of a fabrication process. The higher molecular weight molecules having low comonomer content also have less desirable heat sealing properties. On the other hand, resins containing relatively lower molecular weight molecules, in which the comonomer is invariably concentrated, have better heat sealing properties but tend to exhibit high block and tackiness properties. These lower molecular weight, highly branched molecules also interfere with the proper function of certain additives compounded in the resin, increase the percentage of extractable polymer, and increase fouling in the polymerization plant. The relatively high alpha-olefin comonomer content of these low molecular weight polymer molecules causes such polymer molecules to be generally amorphous and to exude to the surface of fabricated parts, thereby producing an undesirable sticky surface.
Previously known blends of or coextruded polyethylenes designed to improve one or more of the properties of a resulting film or the like relative to its components or relative to polyethylene homopolymers in the past have also suffered from the drawbacks mentioned above. For example, incorporating a blend component with a high average comonomer content to reduce crystallinity and improve heat sealability generally results in an increase of extractables and adversely affects other properties so that the full advantage of the blend is not realized.
Therefore, a need exists for a polymer blend that exhibits improved heat sealing while maintaining other desirable physical properties.